Myocarditis is an inflammatory disease in which the heart becomes injured in the absence of an ischemic event. In the US, most cases of myocarditis are caused by viruses. Several types of viruses have been implicated in myocarditis, but members of the picornovirus family are the most common cause of disease. Previous research, utilizing human and animal models, has demonstrated that both viral- and immune- mediated mechanisms of damage are important in the pathogenesis of myocarditis. The immune system utilizes two distinct pathways to detect viral infections: Toll like receptors (TLR) and cytoplasmic helicases. Specifically, TLR3 and the helicase melanoma-differentiation-associated protein-5 (MDA5) are known to be sensors of dsRNA, a product of viral replication. It has previously been demonstrated that MDA5 is required for protection from picornavirus infection in the heart. Recent studies have shown that TLR3 is also important in this process. The objective of this proposal is to determine the relative contributions of MDA5 and TLR3 to protection from viral myocarditis. We propose that both sensors are important in the prevention of myocarditis and hypothesize that their distinct roles in the disease result from stimulation of distinct patterns of cytokine secretion, or as a result of their expression in different cell types. We intend to test both possibilities by utilizing an in vivo mouse model of myocarditis resulting from infection with the picornavirus encephalomyocarditis virus (EMCV) in mice that are deficient for MDA5 and TLR3. Using this system we will evaluate the cardiac performance, viral replication, and cardiomyocyte survival in the absence of these sensors. Additionally, we will compare the cellular and cytokine innate immune response in WT and sensor-deficient animals and use bone marrow chimeras to investigate the role of MDA5 and TLR3 in the hematopoietic versus stromal compartments. The long term goal of this proposal is to gain a greater understanding of the pathological mechanisms of viral myocarditis in order to understand and potentially control the human disease. Myocarditis is a cardiac disease usually caused by a virus. By understanding the mechanism of how the immune system responds to viral infection in the heart we may be able to develop treatment to limit the course and severity of this disease.